Recently, there has been a strong need to reduce CO2 emissions from fuel combustion for global environmental protection. In particular, there has been a strong need to reduce CO2 emissions from automobiles. Diesel engines are known as internal combustion engines with low CO2 emissions and have already been used as automotive engines. Although diesel engines have low CO2 emissions, they have a problem in that they tend to emit black smoke.
Diesel engines emit black smoke when there is a lack of oxygen for the fuel injected. The black smoke contributes to air pollution and is harmful to humans. Accordingly, it has been attempted to inject fuel into a combustion chamber of a diesel engine at a higher pressure since the injection of fuel into a combustion chamber of a diesel engine at a higher pressure reduces emissions of black smoke. However, injection of fuel into a combustion chamber at a higher pressure requires a fuel injection tube with a higher internal pressure fatigue strength.
To address this need, for example, Japanese Patent No. 5033345 (Japanese Unexamined Patent Application Publication No. 2007-284711) discloses a steel tube for fuel injection that contains, by mass, 0.12% to 0.27% C, 0.05% to 0.40% Si, 0.8% to 2.0% Mn, and at least one of 1% or less Cr, 1% or less Mo, 0.04% or less Ti, 0.04% or less Nb, and 0.1% or less V and that contains, as impurities, 0.001% or less Ca, 0.02% or less P, and 0.01% or less S. The steel tube has a tensile strength of 500 N/mm2 (500 MPa) or more and contains nonmetallic inclusions having maximum diameters of 20 μm or less at least from the inner surface of the steel tube to a depth of 20 μm. JP '345 discloses that the technique allows the injection of fuel into a combustion chamber at a higher pressure to reduce emissions of black smoke while reducing CO2 emissions.
Japanese Patent No. 5065781 (Japanese Unexamined Patent Application Publication No. 2009-19503) discloses a seamless steel tube for fuel injection that contains, by mass, 0.12% to 0.27% C, 0.05% to 0.40% Si, 0.8% to 2.0% Mn, and optionally at least one of 1% or less Cr, 1% or less Mo, 0.04% or less Ti, 0.04% or less Nb, and 0.1% or less V and that contains, as impurities, 0.001% or less Ca, 0.02% or less P, and 0.01% or less S. The steel tube has a tensile strength of 900 N/mm2 (900 MPa) or more and contains nonmetallic inclusions having maximum diameters of 20 μm or less at least from the inner surface of the steel tube to a depth of 20 μm. The technique disclosed in JP '781 involves hardening the steel tube at or above the Ac3 transformation temperature and tempering the steel tube at or below the Ac1 transformation temperature to achieve a tensile strength of 900 N/mm2 or more. JP '781 discloses that the technique prevents fatigue failure initiated from a nonmetallic inclusion present near the inner surface and thus allows for a high critical internal pressure while providing a high tensile strength of 900 N/mm2 or more so that no fatigue occurs when fuel is injected into a combustion chamber at a higher pressure.
JP '345 and JP '781 disclose that the steel tubes contain no nonmetallic inclusions having maximum diameters of more than 20 μm at least from the inner surfaces of the steel tubes to a depth of 20 μm. However, the techniques disclosed in JP '345 and JP '781 have many problems with stable manufacture of steel tubes containing nonmetallic inclusions having maximum diameters of 20 μm or less at least from the inner surfaces of the steel tubes to a depth of 20 μm. Specifically, it is difficult to stably manufacture seamless steel tubes for fuel injection with high strength and good internal pressure fatigue resistance.
It could therefore be helpful to stably provide a seamless steel tube for fuel injection with high strength and good internal pressure fatigue resistance.